This invention is generally related to parenteral fluid warming systems and, more particularly, to the structure of a warming cassette that includes a stiffening frame with an attached fluid container and that is used in a parenteral fluid warming apparatus.
Fluid warming apparatuses, designed to warm and administer parenteral fluids and blood products (hereinafter “fluids”), are in common use. Generally, these fluids are administered using a disposable fluid container which includes a fluid pathway and one or more heat exchange surfaces. The fluid container may be made of plastic film material or thin metal. A warming cassette incorporates such a fluid container, imparting structural support to the container for handling and for being received and supported in the warming unit.
Specifics of a multi-layered fluid container compatible with a supporting cassette frame structure are discussed in applicants U.S. patent application Ser. No. 09/415,558, entitled “PRESSURE TOLERANT PARENTERAL FLUID AND BLOOD CONTAINER FOR A WARMING CASSETTE”, invented by Augustine et al., filed on Oct. 8, 1999, now abandoned.
A warming cassette is placed into a warming unit to heat fluids as they flow through the fluid pathway. Heat is transferred to the fluid through the fluid container by contact with a heat source such as heated metal plates, heated liquid, or heated gas. Metal plate, “dry heat” exchanger warming units are widely known. However, in the last 10 to 15 years, water bath heat exchangers have become the norm in the United States.
While convenient to use, water bath heat exchangers can pose health risks. The warm water in these systems is often circulated for long periods of time without being changed or sterilized. The warm water provides an excellent growth medium for microbes. After several weeks of use, bacteria and fungi can be cultured from these water baths. For these reasons, a “dry heat” system is probably safest for warming medical fluids. However, there are significant fluid thermodynamic problems, as well as convenience, reliability, and cost issues that must be solved for a “dry heat” system to replace the water bath systems.
The American Association of Blood Banks (AABB) mandates that blood products and IV fluids must not be heated above a temperature of 42° C., so as to prevent blood cell damage and thermal injury to a patient. A temperature of 42° C. is easy to maintain under steady-state flow conditions, a low flow rates. However, as the flow rate of the fluid increases, the rate of heat transfer to the fluid must keep pace in order to achieve a target fluid temperature. The boost in the rate of heat transfer is most obviously achieved by using larger heater and by increasing the temperature difference (−T) between the heater and the fluid. Both solutions effectively drive more heat into the fluid. Unfortunately, these solutions are not necessarily effective when the fluid flow rates are highly dynamic. Large heaters and high temperature differentials are not responsive enough to sudden changes in fluid flow rates. For example, in the case of a sudden change from a high fluid flow rate to a low one, the high temperature limit can be exceeded, potentially causing thermal damage to the fluid or patient.
The problems of thermal efficiency and temperature responsiveness over a wide range of flow rates can be met by improving the thermal conductivity of the fluid cassette materials, and minimizing the thickness of the fluid at the point of heat transfer. This implies a thin, flat fluid container, constructed from properly selected materials.
Plastic film materials are commonly used in the manufacture of disposable fluid warming cassettes. However, plastic is a poor heat transfer material. Metal foils, or metal conduits have been used with plastic materials in warming cassettes to enhance thermal conductivity; however, it is difficult to bond metal to plastic materials, and leakage can occur along bonding seams between these materials. Further, metal foils generally increase the cost of cassette manufacturing.
Fluid temperature response may also be improved by reducing the thickness of the fluid channel in the fluid container. In this regard, the space between the heater plates is then reduced to be compatible with thin cassettes. Assume, for example, that an optimal balance between fluid flow resistance and heat transfer for a particular warming unit design yields a distance of 0.048 inches between the heater plates of the unit. It is very difficult to insert an appropriately dimensioned cassette into such a warming unit simply by sliding it between the warming plates. The plastic materials of which such cassettes are made impart little rigidity. Consequently, such a cassette may kink or tear when being slid into or out of such a small space. As a result, “clamshell” solutions have been proposed that spread the warming plates apart when a cassette is inserted or removed from a warming unit.
The limitations of the clamshell design are manifest. Moving parts add to the warming unit's cost, and reduce reliability. It is very difficult to maintain an accurate 0.048 in. spacing across the entire plate surface, when hinges, clasps, and other moving parts are required. Finally, insertions of the cassette into such a warming system becomes a multi-step process, which is both time consuming and inconvenient.
Other problems occur with the use of plastic fluid containers in fixed plate warming units. For example, the fluid channel formed between the plastic films of a fluid container must be contained entirely within the space between the heater plates. However, some portion of the cassette must extend outside of the heater plates in order to provide structure that can be grasped to extract the cassette. If the portion of the cassette that extends outside of that space includes an unsupported portion of the fluid container, the container can rupture when the fluid pressure is increased to increase the flow rate.
It would be advantageous if an efficient and low cost fluid cassette could be developed for a “dry heat” parenteral fluid warming system. Advantage would be gained if the fluid cassette permitted the rapid heating of parenteral fluid under high pressures. Further, it would also be advantageous if the cassette could be made rigid, yet thermally conductive, without the use of metal.
It would be advantageous if a cassette fluid container could be made with plastic walls stiff enough for insertion in between close-set parallel warming plates of a warming unit, yet thin enough to efficiently transfer heat from the plates to the fluid.
It would be advantageous if the above-mentioned cassette could be easily inserted into and removed from the warming unit without being kinked or torn. It would further be advantageous if the cassette had a handle for insertion of the cassette between the warming plates of a fluid warming unit. It would be advantageous if the above-mentioned cassette handle extended outside the unit for convenient handling.